The present invention relates to an apparatus and method for performing magnetic resonance imaging in a preferred fashion and effectively by using a pre-polarizing field.
The principles of magnetic resonance imaging, including the use of a pre-polarizing field, are disclosed in U.S. Pat. No. 4,906,931, naming Raimo Sepponen as inventor, as well as in the corresponding prior Finnish Patent application.
Magnetic resonance imaging is presently a generally applied diagnostic imaging method, which renders results that are superior over other modalities (X-ray, ultrasound). A drawback is the high price of the apparatus, typically 1-3.5 million U.S. dollars. Therefore, it is important to discover methods of bringing the price down. One way is to perform the imaging in lower fields as this is a way of lowering the price of the most expensive component of the apparatus, which is the magnet. However, this impairs the quality of images as the signal they originate from becomes more noisy. Namely, this image signal, which is developed by the resonance of protons in an imaging field B.sub.o, is proportional to the magnetization of protons which in turn is proportional to the magnitude B.sub.o of that field. Thus, for example, if the field is halved, the obtained signal-to-noise ratio (s/n) will also be halved. This can be compensated for by prolonging the imaging time by a factor of four, but in practice this will lead to times that are too long for patients to be imaged.
Since a high quality image is generally considered very important, most of the commercially available equipment is provided with expensive magnets. The most common type is a superconductive magnet, which is capable of producing highest fields, 0.5-2 Teslas, but which are the most expensive to manufacture and operate. The second most common are permanent magnets having a typical field intensity or magnetic flux density of 0.2-0.3 Teslas.
The least popular is an electromagnet, due to the fact that the power and cooling required thereby will be inconvenient at high field intensities. Typically, the power rises up to tens of kilowatts for suitable fields.
The electromagnets can be provided with an air core, as in the figures of the Sepponen patent, or fitted with an iron core. In the latter, an air gap flux passing through the imaging zone is carried from one pole piece to the other by way of a closed, magnetic circuit formed in the iron. An advantage offered by the iron core magnet over one with an air core is a lower power demand, but the greater weight is a drawback. When the field is increased, the weight rapidly increases to tens of tons, since the circulating flux requires everywhere along its route a sufficiently large cross-surface area in order not to exceed the iron saturation flux density, about 1.6-1.8 Teslas.
The Sepponen patent describes a way of constructing an inexpensive and good imaging system by using a so-called pre-polarizing field. Thus, an image is developed in a per se known manner in a homogeneous and stable field B.sub.o, which can be moderately low, for example B.sub.o equal 0.05-0.1 Teslas, whereby it can be produced inexpensively either by an electromagnet or permanent magnet. Prior to the actual imaging event, the field is momentarily increased to a higher value B.sub.p for magnetizing the protons of an object to be imaged. The magnetization occurs exponentially as a function of time, the typical time constants for this being in various human tissues on the order of 0.1-1 seconds, which is thus also a suitable duration for the polarization field. By virtue of pre-polarization, the proton magnetization increases by the factor of approximately B.sub.p /B.sub.o, whereby the s/n ratio is of the same order as in an apparatus which only employs a single field intensity, if such value is selected to be B.sub.p.
The advantages of pre-polarization occurs in two ways: first of all, a high field is only kept on some of the time, which saves power. Secondly, the field B.sub.p need not be particularly homogeneous and this contributes to further saving. In case of air-core electromagnets, field B.sub.p can be created by means of an additional winding which, with the same power, produces more field than the magnet for the field B.sub.o as it can be smaller. This is described in the Sepponen patent.